For many years the control of fuel/air ratio of fuel burners for various furnaces or heating appliances has been desired. One particular method is to automatically search for the peak value (maximum or minimum) of a property of the flame or combustion products which is indicative of the fuel/air or oxidant ratio of the fuel being burned in the burner and by various means adjusting the fuel/oxidant ratio in the combustion chamber for complete combustion.
Several years ago Honeywell Inc. developed an FSP1400 Fuel-Air Ratio Sensor described in a Honeywell publication 95-6957-1 of October 1970 which made use of a flame rod for sensing the ionization current in a small flame having the same premixed fuel/air ratio as the main burner. By means of a control apparatus the fuel/oxidant ratio of the burner was adjusted to provide maximum ionization current. The maximum current always occurred at a premixed fuel/oxidant ratio 15% greater than the stoichiometric ratio. Reducing the fuel/oxidant ratio until the current was 80% of maximum gave stoichiometric combustion.
The present invention is concerned with a gas burning furnace in which the control system searches for and maintains the ionization current at a peak value by controlling the fuel and primary air supply to the burner. This results in an excessive amount of fuel. The secondary air supply to the combustion chamber is controlled proportionally to the primary air supply in such a manner that the fuel/oxidant ratio in the combustion chamber is adequate for complete combustion. Secondary air has little or no effect on ionization current. Other properties of flames or combustion products which have peak values at or near the stoichiometric ratio could also be used to monitor fuel/oxidant ratio. These include flame temperature, flame radiation, H.sub.2 O and/or CO.sub.2 levels in the burned gases, etc. Properties of flames or combustion products which have minimum values at or near the stoichiometric ratio could also be used.